Thermal imaging cameras were originally used to sense heat or aid night vision. For vision applications there was no need for absolute temperature measurement. As these cameras started being used for process control, however, manufacturers started making cameras with absolute temperature measurement capabilities. Using the same conversion schemes used by spot infrared pyrometer manufacturers, camera manufacturers built in schemes to provide actual temperature measurements. They counted on the output of the detector to follow Planck's equation when looking at black body targets.
As the use of thermal imaging cameras progressed, imaging cameras were being used in new applications requiring a faster detector response and cameras able to handle higher temperature ranges. These higher temperature ranges and faster response times, however, are not suited for traditional infrared pyrometer detectors due to their slow response times and low temperature thresholds. In contrast, a thermal imager with an InGaAs detector does have a quick response time and is suitable for higher temperature ranges.
The InGaAs detector array integration readout circuitry, however, creates a non-linearity so that the output does not follow the predicted Planck curve. Further, since real targets are not black bodies, it is necessary to compensate the detector output with an emissivity correction that is accomplished when the output tracks the Planck curve. When it does not, as is the case with the InGaAs array and its associated readout circuitry, correcting for emissivity is a complicated matter which prior to now was not been adequately resolved.
While the method and device described herein are susceptible to various modifications and alternative constructions, certain illustrative embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the disclosure and the appended claims.